Radio communication devices use antennas to provide for the efficient transmission of radio frequency (RF) communication signals. The transmitter portion of a communication device includes a power amplifier to amplify the radio frequency signals before they are coupled to the antenna for transmission. As modern radio communication systems work in narrow frequency bands the transmitters' circuitries require RF power amplifiers able to operate in a linear fashion. Linear amplification is required to prevent distortion of the modulated signal and minimizing the interference. However non-linearity of real world RF amplifiers appears when they are operated at high drive levels. Similar situations may be caused by operating conditions. For example, a transmitter operating near an electromagnetically reflective structure may be susceptible to energy reflected back through the antenna into the transmitter.
There are known in the art transmitters with improved linearity. One method of linearization of transmitters is to use a Cartesian feedback loop based linearizer. The Cartesian feedback linearizer allows maintaining linearity of the transmitter while still allowing RF power amplifier to work close to its saturation point thus maintaining good efficiency. To protect against changes in load impedance as a result of reflected energy, an isolator or circulator is often inserted between the antenna and the power amplifier. The isolator protects the power amplifier by absorbing the reflected energy and preventing it from reaching the amplifier. The isolator directs the reflected energy to an absorptive load termination. Although the isolator generally works well, it adds significant cost, size, and weight to the design of a radio communication device. Isolators are narrowband, expensive and have large physical dimensions (especially at low frequencies).
There are also known in the art Cartesian loop transmitters without isolators. One such example is described in US patent application no. US2003/0031271. In this document a method for isolator elimination is disclosed. In this prior art solution an isolator eliminator provides phase and level correction signals on the basis of samples of an information signal and a drive signal sampled from a feedback loop. These correction signals maintain stability the operation of the transmitter.